An illumination optical system and a projection display apparatus include a beam combining portion (4) and a condensing lens (8) condensing a beam from the beam combining portion. The beam combining portion combines beams from first and second lamp units each including a lamp and a concave mirror, the lamp having a cathode and an anode arranged along an optical axis of the concave mirror. The beam combining portion includes a first reflective section (31B) disposed on an optical axis of the first lamp unit and off the optical axis of the second lamp unit and a second reflective section (32B) disposed off the optical axis of the first lamp unit. The first and second reflective sections reflect a beam that is emitted from the second lamp unit and that is off an optical axis thereof to combine the beams from the first and second lamp units.